Method and apparatus for the improved combustion of biomass fuels

ABSTRACT

A cylindrical furnace having a vertical axis controls combustion. Solid fuel, particulates, and gases inside the furnace rotate around the axis, inducing radial stratification using centrifugal forces. Fuel and particulates drag on the wall of the cylinder, slipping in and out of suspension, thereby increasing particle residence times. The solid particles comprise combustible fuel particles, and non-combustible ash and contaminants. Control of the temperature of non-combustible particles and the wall surface prevents these non-combustible particles from adhering to, and building up on, the furnace wall. It is also advantageous to control the gas temperature leaving the furnace to minimize temperature-driven corrosion of downstream heat-exchange surfaces. Method and apparatuses are described to control the gas, non-combustible particle, and wall temperatures. The furnace can be integrated into a stand-alone boiler or as a combustor in which a portion of the pyrolysis gas from the combusting fuel is burned in a separate vessel.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to solid fuel furnaces, and isparticularly applicable to furnaces that burn biomass fuels.

BACKGROUND OF THE INVENTION

In many power production, agricultural, and industrial processapplications, biomass fuels are burned to produce steam, heat, andpower. The fuel is typically burned in a furnace that is part of aboiler. In that case the hot gas from the furnace will typically passacross heat absorbing tubes, filled with water or steam, such as ascreen, superheater, generating bank, and economizer. These tube bundlesmake up the convective section of the boiler. The furnace walls aretypically constructed of water-filled tubes but absorb heatpredominantly through radiation. The biomass fuels may include, but arenot limited to bark, biological sludge, clippings (branches), forestresidues, fiber line rejects (knots and screen rejects), urban waste(construction debris, pallets), agricultural waste (straw, rice husks,palm oil shells), sugar cane bagasse and pith, and sugar cane trash(tops and leaves). Biomass fuels are difficult to burn because they areoften very wet (50% or higher moisture content), may have low specificdensity, may contain abrasive contaminants and corrosive chemicals, andmay require significant processing or treatment to facilitatecombustion.

In a typical furnace, it is desirable to create turbulent mixing of thecombustion air, fuel, and pyrolysis gases but the turbulence is ofteninsufficient to effectively burn the fuel. For example, if a fuel has acombination of high moisture content and low specific density (thedensity of individual fuel particles), the fuel can become entrained inthe gas flow and exit the furnace before it has time to burn completely.If the fuel cannot be burned quickly, the combustion may continue intoundesirable areas of the furnace or boiler. Light and wet fuels such assugar cane bagasse are readily entrained in the gas flow and may exitthe furnace before they burn completely. This can cause overly hightemperatures in the convective sections of the boiler or even furtherdownstream, and in the worst cases, combustion may continue into theeconomizer or beyond. The gases in an industrial furnace typically havea residence time of 2-3 seconds (the duration that air and combustiongases remain within the volume of the furnace) and, if the fuel isentrained in the gas flow, that is often not enough time to complete thecombustion of wet and light fuels.

Grate-fired boilers are conventionally used for burning biomass fuels.These boilers may have a traveling grate, vibrating grate, or fixedgrate (sloping, horizontal, or stepped). The fuel is injected or dumpedonto the grate and combustion air is blown up through the grate(under-grate air or sometimes called primary air) to burn the fuel. Insome of these boilers the under-grate air may constitute up to 80% ofthe total combustion air as a lot of air is needed to cool the grate. Inmany grate-fired boilers the fuel does not burn uniformly across thegrate due to variations in fuel distribution, moisture content, sizedistribution, air distribution, etc. Where the fuel does burn, the fueldepth on the grate is reduced and the under-grate air, taking the pathof least resistance, bypasses the deeper fuel and further increases thecombustion in the shallow areas thereby exacerbating the poordistribution of combustion across the grate. Because of this mechanismthe surface area of the grate is poorly utilized and a lot of fuel oftengoes unburned and wasted, reducing the efficiency of the boiler andcreating disposal challenges of the ash.

Bubbling fluidized bed boilers (BFBs) are also commonly used for burningbiomass fuels. In a BFB, a sand bed is typically fluidized using primaryair and fuel is dumped into or injected over the bed. The sand acts as aheat sink and mixing medium and ideally, the fuel is distributeduniformly through the bed and mixes with the primary air, dries, andburns. BFB boilers, while being advantageous for burning wet fuels havesignificant drawbacks. They can be finicky to operate as the temperatureand fluidization of the bed must be carefully controlled or the sand mayoverheat and sinter or glassify in the worst cases. BFBs are alsosusceptible to erosion caused by the entrainment of fine sand particlesin the gas stream. The entrained particles can, in effect, sandblast theconvective tube surfaces causing premature degradation and failure.

A common feature of most boilers used to burn biomass fuels is the largeamount of under-grate or primary air required, as described above. Toefficiently burn any fuel, it is desirable to minimize the excess air(the amount of air used above the stoichiometric requirement) but thisis dependent on good mixing of the air and fuel. Over-fired air (OFA)systems are commonly used to improve the mixing and combustion inbiomass boilers. OFA systems inject combustion air at one or moreelevations above the grate or fluidized bed to complete the mixing ofthe fuel (entrained fuel particles and pyrolysis gases). If a boileruses a high percentage of primary air, however, there may not be enoughair available for good mixing at the OFA levels.

Alternative boiler designs have been developed that require much lessprimary air, for example, the stepped-floor grate as described in U.S.Pat. No. 8,707,876, the V-cell grate system described in U.S. Pat. No.9,140,446, and the cylindrical power boiler described in U.S. PatentPublication No. 2016/0195260. U.S. Pat. No. 8,707,876, U.S. Pat. No.9,140,446, and U.S. Patent Publication No. 2016/0195260 are owned byApplicant and are all hereby incorporated by reference. These systemsare designed, at least in part, to enable the operation of a boiler withless primary air thereby freeing up more air to be injected above thefuel bed where it can more effectively improve the combustion of thefuel so that the boiler can operate more efficiently.

The power boiler described in U.S. Patent Publication No. 2016/0195260is particularly useful for burning wet and light fuels such as sugarcane bagasse, pith, and trash. Sugar cane is the largest agriculturalcrop grown in the world (by area planted) and generates a tremendousamount of waste biomass. Bagasse and pith are commonly burned inoutdated grate fired boilers that suffer from the deficiencies describedabove. Sugar cane trash, the left-over leaves and tops of the plants, isonly minimally burned because the chlorine content of the leaves cancause corrosion of the high temperature superheater tubes inconventional boilers. Historically, most of the cane trash was burned inthe fields but this practice is being curtailed around the world toreduce air pollution. This has created a dilemma for the sugar canegrowers but also an opportunity to utilize an abundant, renewable fuelfor power production.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved furnace forburning biomass fuels.

A furnace configured for burning biomass fuel includes a cylindricalenclosure with a vertical axis, the top of said enclosure disposed toconduct hot gases to heat-absorbing surfaces or to a separate locationfor further combustion. Solid fuel particles and air are injected intosaid furnace and said fuel burns releasing gaseous products ofcombustion, in which noncombustible particles are present, in which saidgases are induced to rotate about the axis of said cylindricalenclosure. Said rotation entrains at least some of said fuel andnoncombustible particles to rotate with said gases, and the averageresidence time of said particles in said cylindrical enclosure isgreater than the average residence time of said gases in saidcylindrical enclosure. The air-to-fuel ratio is controlled at least atone combustion air injection elevation to control a temperature abovethe elevation of said combustion air injection.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the scope of the invention as set forthin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional elevation of a furnace depicting the controlelements

FIG. 2 shows is a sectional plan view through the furnace showing thespatial relationship of some of the described features. The arrows inFIG. 2 show the direction of flow of fuel and gas.

FIG. 3 is a flow chart showing a process of combusting biofuels.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Chloride corrosion in biomass-fired power boilers is a temperaturedriven phenomenon with higher temperatures accelerating corrosion rates.For this reason, superheater tubes are more susceptible to chloridecorrosion as they operate at higher temperatures. Also, unburned carbon(unburned fuel) leaving the furnace may be trapped in the convectivesections and burn there. This can create a high temperature, localizedreducing environment that can accelerate corrosion. Another problem withconventional boilers is sintering of ash and inorganic contaminants(predominantly sand) in the boiler. Sintering of these materials is alsoa temperature driven phenomenon and their agglomeration can adverselyaffect the operation of the boiler. For example, if a boiler has acylindrical furnace as described in U.S. Patent Publication No.2016/0195260, and operates by spinning the gases inside the furnace topush entrained particles to the perimeter, a build-up of ash and/or sandon the wall can impede the circulation of gas in the furnace.

Complete combustion and good temperature control throughout the furnacefacilitates operation of furnaces that depend on the interaction of thegas flow and the wall(s) of the furnace and also facilitates efficientcombustion of biomass fuels, especially sugar cane trash and otherchlorine-containing fuels. Good control in turn requires reducing theprimary air so that enough air is available in the OFA system toregulate the combustion throughout the furnace. One preferred embodimentutilizes a cylindrical furnace (US application 2016/0195260) with aV-Cell floor (U.S. Pat. No. 9,140,446) and an automated control systemto ensure complete combustion, to control gas temperature, and preventexcessive buildup on the furnace wall and corrosion of boiler tubes.

FIG. 1 is a sectional elevation of a furnace depicting the controlelements. Solid lines with arrows indicate process connectionstransporting fuel or gases and the arrows indicate the direction offlow. Dashed lines with arrows indicate control connections and thearrows indicate the direction of the flow of information.

Referring to FIG. 1, furnace 1 has a cylindrical form about verticalaxis 2. The furnace bottom 3 is shaped as a truncated cone which isterminated at a lower end at a floor which may comprise a gate 25, andthe furnace top 4 is disposed to conduct hot gas to downstream heattransfer surfaces or a tandem vessel for further combustion. The heattransfer surfaces may be integrated with the furnace to form astand-alone boiler, or the gas may be conducted to a separate boiler forfurther combustion or to a separate heat recovery steam generator. Fuelis stored in fuel bin 5 and delivered through fuel chute 6 and feedermeans 7 and is then pneumatically conveyed to the furnace throughinjection pipe 8. The motive force for the fuel injection is provided byflue gas fan 9 and booster fan 10, supplying recirculated flue gas topneumatically inject the fuel. The fuel may be injected into the furnaceat multiple locations and elevations. Flue gas flow control damper 11controls the flow of flue gas to feeder means 7. Combustion air fan 12supplies air through distribution duct 13 to OFA (Over Fired Air) nozzleduct 14 through air flow control damper 15. Flue gas fan 9 supplies fluegas through distribution duct 16 to OFA nozzle duct 14 through flue gasflow control damper 17. OFA Nozzle duct 14 is connected to injectionnozzle 18, that is fitted with pressure control damper 19. Although oneinjection nozzle 18 is shown, additional similar nozzles may be suitablylocated at multiple locations and elevations around furnace 1. Fuel bedair duct 20 is connected to furnace bottom 3 and supplies combustion airthrough air flow control damper 21 or flue gas through flue gas flowcontrol damper 22, or a mixture of the two, to cooling plenum 23. Theair, flue gas, or mixture, then flows down through cooling plenum 23 toupper ash chute 24, then up through movable grate 25 as shown by arrows26. Movable grate 25 is perforated to allow gas to pass therethrough.Agitation steam pipe 27 supplies steam through steam flow control valve28 to an array of nozzles 29 around the inside of furnace bottom 3.Upper ash chute 24 is closed at the bottom 3 by upper ash damper 30,thus forcing the air and/or gas flowing into upper ash chute 24 fromcooling plenum 23 to flow up through ash grate 25. Lower ash damper 31creates an air lock between itself and upper ash damper 30. Ash grate 25is opened temporarily to discharge ash into ash chute 24, then returnsto its closed position. Upper ash damper 30 then opens temporarily todischarge the ash to lower ash chute 32, then returns to its closedposition. Lower ash damper 31 then opens temporarily to discharge theash from the furnace. During normal operation, upper ash damper 30 andlower ash damper 31 are never open at the same time, therefore the airand/or gas flowing from cooling plenum 23 is always forced to flow upthrough movable grate 25 and cannot leak out of the furnace.

Fuel is fed from fuel bin 5 to fuel chutes 6 by fuel feed screw 33,powered by electric motor 34, and the speed of motor 34 is controlled byVFD (variable frequency drive) 35. One fuel feed system is showncomprising items 6, 7, 8, 33, 34, and 35, but multiple similar fuel feedsystems may be used. Flue gas fan 9, booster fan 10, and combustion airfan 12 are powered by motors 36, and the speed of flue gas fan 9,booster fan 10, and combustion air fan 12 are controlled by VFDs 37, 38,and 39 respectively. Flue gas flows from flue gas fan 9 through duct 40to booster fan 10, then from booster fan 10 through duct 41 to flue gasflow control damper 11.

A gas cooling means is shown at 81 and employs a water spray to reducethe gas temperature. Control valve 82 is regulated to inject the desiredamount of water. Multiple water sprays may be suitably located withinthe furnace. For example, the gas temperature tends to be hottest justdownstream of each injection nozzle 18 where combustion air is injected.A water spray can be located adjacent to each nozzle to control the peakgas temperature. Other ways to cool the gas can also be used. Forexample, steam injection can be used in lieu of, or in conjunction with,a water spray but a water spray will absorb more heat due to the latentheat of vaporization.

Ash extraction duct 42 is located in the upper half of furnace 1 andpulls a portion of the ash-laden flue gas out of the furnace. Cyclone 43separates the ash from the flue gas and the gas is returned to mix withthe remaining flue gas downstream, as shown at 44. The ash is retainedin cyclone hopper 45 until it is periodically discharged through airlock valves 46. The ash may be discharged to a disposal means or may becollected for analysis to determine, for example, the chlorine contentor unburned carbon content. An elevated unburned carbon content of theash may indicate the combustion needs to be improved and the chlorinecontent can be used to determine an optimum operating temperature tominimize corrosion and plugging of downstream convective sections.Analyzing the content of the extracted fly ash may be automated or maybe an offline endeavor, but temperature element 47 and temperaturetransmitter 48 can be used to continuously monitor the temperature ofthe extracted ash. This will provide feedback to furnace controller 49to adjust the combustion in the boiler to, for example, keep the ashtemperature below a predetermined set point. One ash extraction systemcomprising items 42-48 is shown but multiple similar ash extractionsystems may be used.

Temperature element 50 and temperature transmitter 51 may be used aspart of temperature measuring system 52 to measure the average furnaceexit gas temperature (FEGT) across the width of the furnace 53. Anacoustic pyrometer may be suitably employed as measuring system 52.Temperature transmitter 51 will send a feedback signal to furnacecontroller 49 to adjust the combustion in the boiler to maintain theFEGT within a predetermined or calculated range. One temperaturemeasuring system comprising items 50-52 is shown but multiple similartemperature measurement systems may be used. Temperature element 50, andother temperature elements, may be any appropriate type of temperaturesensor.

FIG. 2 shows is a sectional plan view through the furnace showing thespatial relationship of some of the described features. The arrows inFIG. 2 show the direction of flow of fuel and gas. Referring to FIG. 2,fuel enters the furnace through at least one injection pipe 8. Flue gasis used to convey the fuel and inject it into the boiler and enters withthe fuel. Air, flue gas, or a combination thereof, is injected throughat least one injection nozzle 18. Injection pipes 8 and injectionnozzles 18 are aligned tangentially with imaginary circle 54 inside thefurnace. One imaginary circle is shown but injection pipes 8 andinjection nozzles 18 may be tangential to different imaginary circles.Similar arrangements of injection pipes 8 and injection nozzles 18 canbe replicated at multiple elevations in the furnace. Utilizing injectionpipes 8 and injection nozzles 18 in this manner will induce the fuel,air, and combustion gases to rotate about vertical axis 2 inside thefurnace. The rotation will impart a centrifugal force to the fuel andgases, separating the denser fuel and denser gas (generally cooler) tomove to the perimeter of the furnace and the less dense gas (generallyhotter) to be displaced to the center. This will cause the coolercombustion air to mix with the fuel around the perimeter and the fuelparticles will drag on the furnace wall where they will slip in and outof suspension in the gas. This will cause the average retention time ofthe fuel to be greater than the average retention time of the gasthereby giving the fuel more time to be burned completely.

As the biomass fuel burns, the residual inorganic ash can be abrasiveand biomass fuels typically contain significant amounts of contaminatingsand, both of which can become entrained in the swirling gas flow andabrade the wall tubes of the furnace. Left unchecked this abrasion willquickly erode the tubes causing the furnace or boiler to shut down. Toprevent erosion of the wall tubes, a common practice is to line thefurnace with a high-temperature abrasion-resistant refractory. Inaddition to protecting the tubes, the refractory also acts as a heatsink and helps to stabilize the combustion in the furnace byalternatively absorbing and releasing heat as the combustion in thefurnace naturally fluctuates. The refractory lining has inherentdisadvantages, however, in that it impedes the radiant heat transfer tothe water-filled wall tubes and the elevated temperature of therefractory surface can cause the ash and sand circulating with the gasto stick to the furnace wall. The cylindrical furnace depends on therotation of the internal gases to increase the retention time of thefuel and to strip contaminating ash and sand out of suspension tominimize the erosion and pluggage of downstream tube banks. Ifsufficient material accumulates on the furnace wall, the necessarycirculation of the gases will be impeded, and the furnace will notoperate properly.

The stickiness of the ash and sand are determined by their meltingbehavior and that in turn is influenced by their chemical makeup. Thechlorine content of the ash, for example, acts as a eutectic with higherchlorine content depressing the melting temperature of the ash. Themelting behavior is characterized by four stages: first melting, stickyrange, radical deformation, and complete melting, with the temperatureincreasing from the first to the last stages respectively. For the ashand sand or other significant inorganic contaminants, laboratory testswill be conducted to determine the different melting stage temperaturesand that data will be used as variables in the programming of furnacecontroller 49. Similarly, a sintering temperature may be determined forthe ash or sand with the sintering temperature indicating a stickytemperature. Some furnaces are designed to operate at temperatures abovethe radical deformation temperature of the ash. These are calledslagging furnaces and at those temperatures the ash is fluid enough torun off under its own weight. In biomass burning furnaces, however,especially those burning fuels with high chlorine content, elevatedtemperatures can increase the chlorine-induced corrosion of the tubesurfaces. To minimize the potential for chlorine induced corrosion, tominimize the excrescent material build-up on the furnace wall, and tominimize the pluggage of any convective tube banks, the furnace isdesigned to operate below the sticky range of the ash and contaminatingsand. This requires strict control of the combustion in the boiler.

Referring again to FIG. 1, furnace 1 is lined with refractory 55 and atleast one thermowell 56 is imbedded in refractory lining 55. In someembodiments, multiple thermowells are located at suitable locationsaround the perimeter of the furnace and at multiple elevations withinthe furnace. Temperature element 57 (typically a thermocouple) willcollect temperature data and temperature transmitter 58 will send thedata to furnace controller 49. Furnace bottom 3 is lined with refractory59 but refractory lining 59 may have different physical characteristicsthan refractory lining 55. At least one thermowell 60 is imbedded inrefractory lining 59, but in some embodiments, multiple thermowells aresuitably located within furnace bottom 3. Temperature element 61(typically a thermocouple) will collect temperature data and temperaturetransmitter 62 will send the data to furnace controller 49.

Furnace controller 49 uses a distributed control system, or aprogrammable logic controller, or another computer-based system tocollect and analyze input data and send out control signals to variouscontrol elements around the furnace. FIG. 1 does not show all the inputsand outputs to and from furnace controller 49, only some of thoseassociated with controlling the combustion temperature. Many controlscenarios are possible; several are described below:

Control Scenario #1: Thermowell 56 is located above injection nozzle 18.The combusting gases are circulating in the furnace but following ahelical path upward, therefore the temperature is most suitably measuredabove the associated control element. If the temperature input fromtemperature transmitter 58 to furnace controller 49 is higher thandesired, furnace controller 49 can decrease the combustion air flowthrough injection nozzle 18 by closing air flow control damper 15. Thiswill reduce the air to fuel ratio at that elevation, the combustion willbe retarded, and the refractory temperature measured at thermowell 56will drop. Similarly, if the temperature is too low the air to fuelratio can be increased. Thermowell 56 measures the temperature of therefractory in which it is embedded close to the interior surface of therefractory, preferably within 10 millimeters. The most accurate means todetermine the gas temperature based on the temperature measurement atthe thermowell is to independently measure the gas temperature using ahand-held pyrometer or by other means, and establish a correlationbetween the gas temperature and the refractory temperature. Alternately,a calculation can be made to determine the gas temperature based on therefractory temperature measurement, tube temperature, refractoryproperties, location of the thermowell, and thickness of the refractory.At the location of the thermowell, the refractory will be cooler thanthe combustion gas temperature as it is cooled by the water filled tubessurrounding the vessel, and the heat transfer from the gas to therefractory is imperfect. For example, typical sticky temperature forsugar cane trash is around 1000° Celsius. If the average gas temperatureis 900° C., and the tube temperature is 227° C., and silicon carbiderefractory is used with an average thickness of 75 mm, the surfacetemperature of the refractory can be calculated to be about 616° C. Oncethat is known, the temperature profile through the refractory can beestablished. For example, with the parameters given, the refractorytemperature at the thermowell may be about 564° C. Now, the heattransfer to the refractory is predominantly driven by radiation and isgenerally a function of the difference between the gas temperature tothe 4th power and the refractory surface temperature to the 4th power.So, any increase in the gas temperature will dramatically increase theheat flux to the refractory and the refractory temperature will riseaccordingly. In this control scenario, therefore, if the temperature atthe thermowell rises above 564° C., the control system will start toclose the associated damper to retard the combustion to prevent the gastemperature from reaching the sticky point.

Control Scenario #2: Ash extraction duct 42 is located above injectionnozzle 19. The temperature of the particulates extracted by extractionduct 42 is measured by temperature element 47 and transmitted to furnacecontroller 49 by temperature transmitter 48. If the measured temperatureof the particulates is higher than the sticky temperature of the ash,for example, furnace controller 49 can reduce the combustion air flowingto injection nozzle 18 by closing air flow control damper 15. This willreduce the air-to-fuel ratio at and above the elevation of injectionnozzle 19, retard the combustion in the furnace above that point, andreduce the temperature of the particulates captured by ash extractionduct 42.

In either Control Scenario #1 or #2 it may be desirous to maintain acertain total gas flow through injection nozzle 18 to maintain therotation of the gas inside the furnace. In that case, furnace controller49 can adjust the air to fuel ratio at and above the elevation ofinjection nozzle 18 by opening (or closing) flue gas control damper 17and simultaneously closing (or opening) air flow control damper 15.

In either Control Scenario #1 or #2, if the flow through injectionnozzle 18 changes, the position of pressure control damper 19 can beadjusted to increase or decrease the upstream gas pressure in injectionnozzle 18. For example, if the flow decreases, pressure control damper19 can be closed to increase the pressure. This will increase the gasvelocity leaving injection nozzle 18 and maintain the momentum transferfrom the injected gas to the rotating gas inside furnace 3. In this casepressure transmitter 63 will provide a feedback signal to furnacecontroller 49.

Temperature measuring system 52 is employed to measure the average gastemperature leaving furnace 1. Temperature transmitter 51 sends themeasured temperature to furnace controller 49 that in turn controls theair to fuel ratio in furnace 1 by adjusting air flow control damper 15,flue gas control damper 17, and/or pressure control damper 19.

If multiple gas injection systems, similar to items 14, 15, 17, 18, and19, are installed around the boiler, they may be controlled in parallelor individually to control the combustion temperature locally orthroughout the furnace while maintaining the desired rotation of thecombusting gas in the furnace.

The temperature measured at 56 by temperature element 57 is sent tofurnace controller 49 where it is compared against a desired value. Ifthe temperature at 56 is found to be too high, furnace controller 49 mayopen valve 82 and inject a flow of water into the furnace. Furnacecontroller 49 can turn the flow of water on or off, or regulate theflow, to control the temperature at 56. Water spray 81 can be used tocontrol a local temperature such as at 56, or an average gastemperature, such as that measured by measurement system 52. In thelatter case, multiple water sprays 81 may be suitably employed.

Using temperature element 57 to measure the internal temperature ofrefractory lining 55 is a practical, reliable, and well-establishedprocedure, but to avoid buildup of ash and contaminants on the furnacewall, it is necessary to know the surface temperature of refractorylining 55. This may be calculated given the temperature measured bytemperature element 57, thickness of refractory lining 55, the locationof thermowell 56, the temperature of the furnace wall tubes 64, and thethermal conductivity of refractory lining 55. It is also advantageous tomeasure the temperature of the gas close to the surface of refractorylining 55. The gas temperature is difficult to calculate accurately fromthe refractory temperature, but it can be measured directly by a varietyof means. Gas pyrometer 65 may comprise an optical pyrometer, a laserpyrometer, a suction pyrometer, a thermocouple exposed to thecirculating gas, or other technology. Gas pyrometer 65 is ideallylocated above injection nozzle 18 to measure the gas temperatureslightly downstream from injection nozzle 18. Temperature transmitter 66sends the temperature data to furnace controller 49. FIG. 1 shows onegas pyrometer, but multiple pyrometers may be installed at differentelevations and locations around the furnace.

Additional instrumentation may also be used to control the operation ofthe furnace. Optical camera 67 can be used to ascertain the depth,movement, and combustion of the fuel bed at the bottom 3 of furnace 1,or it can be used to determine the rotational velocity of the gases inthe furnace. Feedback can be through manual observation by a humanoperator or by a machine vision system providing feedback to furnacecontroller 49. Multiple optical cameras may be used to monitor separateaspects simultaneously. Flow elements 68, 70, 72, and 74, and flowtransmitters 69, 71, 73, and 75 can be used to measure and transmitcombustion air and flue gas flow data to furnace controller 49. Pressuretransmitters 76 can be used to measure and transmit the gas pressures inbed air duct 20 to furnace controller 49. Furnace controller 49 can usethat data to adjust injection nozzle damper 19 to maintain the desiredinjection velocity. Pressure transmitters 77, 78, and 79 can transmitthe discharge pressures from the combustion air fan, flue gas fan, andbooster fan, respectively, to furnace controller 49. Furnace controller49 can use that data to control the speed of combustion air fan 12, fluegas fan 9, and booster fan 10 to maintain their desired dischargepressures. Pressure transmitters 76 and 80 can provide feedback tofurnace controller 49 to control the pressure in bed air duct 20 andsteam pipe 27 respectively.

Furnace controller 49 may be programmed to operate the boiler asautonomously as possible but some human intervention may be required.For example, periodic observations by human operators can fill in gapsin operational data not obtainable by an automated system. There alsomay be times when field instrumentation is offline and human operatorsmust fill in. Human observations may therefore form part of a feedbackloop to furnace controller 49.

The furnace is limited to use in a boiler and can be used in otherapplication in which a solid fuel is burned, such as in a gasifier, inwhich gaseous combustion products are removed from the furnace andburned in a separate location. While several embodiments are disclosed,one skilled in the art will recognize that many other embodiments arepossible within the scope of the invention.

FIG. 3 is a flow chart 300 showing a process of combusting biofuels. Inblock 302, a mixture of solid fuel particles and combustion air isinjected into a cylindrical furnace, such as a furnace described inFIGS. 1 and 2. The fuel-air mixture from block 302 is then burned inblock 304. The configuration of air and fuel injectors shown in FIG. 2then induces rotation of the fuel-air mixture around an axis of thecylindrical furnace in block 306. In block 308, radial centrifugalforces occurring due to the rotation of the fuel-air mixture induce aradial stratification wherein the heavier solid particles tend toconcentrate near the wall of the cylindrical furnace while the lightergases remain nearer the axis. In block 310, the particles which areconcentrated near the wall of the cylindrical furnace tend to rotateslower and remain within the furnace longer than the combustion gases.In block 312, a temperature at or near an air injection location ismeasured and then in block 314, this measured temperature is compared toa desired temperature range. Block 316 is entered if the temperature isabove the desired range (i.e., T too high)—the flow of combustion air isthen reduced to decrease the amount of combustion and thereby lower thetemperature at and above the air injection location. Block 318 isentered if the temperature is below the desired range (i.e., T toolow)—the flow of combustion air is then increased to increase the amountof combustion and thereby raise the temperature at and above the airinjection location. If block 314 indicates that the temperature iswithin the desired range, then block 312 is re-entered to continuetemperature measurements.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable. The invention has broadapplicability and can provide many benefits as described and shown inthe examples above. The embodiments will vary greatly depending upon thespecific application, and not every embodiment will provide all of thebenefits and meet all of the objectives that are achievable by theinvention.

It should be recognized that embodiments of the present invention can beimplemented via computer hardware, a combination of both hardware andsoftware, or by computer instructions stored in a non-transitorycomputer-readable memory. The methods can be implemented in computerprograms using standard programming techniques—including anon-transitory computer-readable storage medium configured with acomputer program, where the storage medium so configured causes acomputer to operate in a specific and predefined manner—according to themethods and figures described in this Specification. Each program may beimplemented in a high-level procedural or object-oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits programmed for thatpurpose.

Further, methodologies may be implemented in any type of computingplatform, including but not limited to, personal computers,mini-computers, main-frames, workstations, networked or distributedcomputing environments, computer platforms separate, integral to, or incommunication with charged particle tools or other imaging devices, andthe like. Aspects of the present invention may be implemented in machinereadable code stored on a non-transitory storage medium or device,whether removable or integral to the computing platform, such as a harddisc, optical read and/or write storage mediums, RAM, ROM, and the like,so that it is readable by a programmable computer, for configuring andoperating the computer when the storage media or device is read by thecomputer to perform the procedures described herein. Moreover,machine-readable code, or portions thereof, may be transmitted over awired or wireless network. The invention described herein includes theseand other various types of non-transitory computer-readable storagemedia when such media contain instructions or programs for implementingthe steps described above in conjunction with a microprocessor or otherdata processor. The invention also includes the computer itself whenprogrammed according to the methods and techniques described herein.

Computer programs can be applied to input data to perform the functionsdescribed herein and thereby transform the input data to generate outputdata. The output information is applied to one or more output devicessuch as a display monitor. In preferred embodiments of the presentinvention, the transformed data represents physical and tangibleobjects, including producing a particular visual depiction of thephysical and tangible objects on a display.

Whenever the terms “automatic,” “automated,” or similar terms are usedherein, those terms will be understood to include manual initiation ofthe automatic or automated process or step.

In the description and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” To theextent that any term is not specially defined in this specification, theintent is that the term is to be given its plain and ordinary meaning.The accompanying drawings are intended to aid in understanding thepresent invention and, unless otherwise indicated, are not drawn toscale.

The various features described herein may be used in any functionalcombination or sub-combination, and not merely those combinationsdescribed in the embodiments herein. As such, this disclosure should beinterpreted as providing written description of any such combination orsub-combination.

The following are additional enumerated embodiments according to thepresent disclosure.

A first embodiment, which is a furnace burning biomass fuel defined by acylindrical enclosure with a vertical axis, a top of said enclosuredisposed to conduct hot gases to heat-absorbing surfaces or to aseparate means for further combustion, in which solid fuel particles andcombustion air are injected into said furnace and said fuel burnsreleasing gaseous products of combustion, in which noncombustibleparticles are present, in which said gases are induced to rotate aroundthe vertical axis and said rotation entrains at least some of said fueland noncombustible particles to rotate with said gases, and an averageresidence time of said particles in said cylindrical enclosure isgreater than an average residence time of said gases in said cylindricalenclosure, and in which an air-to-fuel ratio is controlled at least atone combustion air injection elevation to control a temperature abovethe elevation of said combustion air injection.

A second embodiment, which is the furnace of the first embodiment inwhich at least one melting temperature has been determined for at leastone composition of said noncombustible particles and said meltingtemperature is used to determine a desired range of said controllabletemperature.

A third embodiment, which is the furnace of the second embodiment inwhich said cylindrical enclosure is internally lined with refractory andsaid controllable temperature is the temperature of said refractorymeasured at least at one location within said cylindrical enclosure.

A fourth embodiment, which is the furnace of the second embodiment inwhich said controllable temperature is a furnace exit gas temperaturemeasured at least at one location at an exit of said cylindricalenclosure.

A fifth embodiment, which is the furnace of the second embodiment inwhich said controllable temperature is a temperature of saidnoncombustible particles extracted from said cylindrical furnace.

A sixth embodiment, which is the furnace of the second embodiment inwhich said controllable temperature is a temperature of said gasmeasured within a cylindrical height of said furnace.

A seventh embodiment, which is the furnace of the first embodiment inwhich said air-to-fuel ratio is controlled by regulating a quantity ofsaid combustion air injected at least at one of said air injectionelevations.

An eighth embodiment, which is the furnace of the seventh embodiment inwhich an injection velocity of said combustion air is controlledindependently of a flow of said combustion air.

A ninth embodiment, which is the furnace of the first embodiment inwhich said air-to-fuel ratio is controlled by diluting said combustionair with oxygen-depleted gas taken from a furnace flue gas.

A tenth embodiment, which is the furnace of the ninth embodiment inwhich an injection velocity of said diluted combustion air and saidfurnace flue gas is controlled independently of a flow of said dilutedcombustion air and said furnace flue gas.

An eleventh embodiment, which is a furnace burning biomass fuel definedby a cylindrical enclosure with a vertical axis, a bottom of saidenclosure being conically shaped and truncated at a floor, a top of saidenclosure disposed to conduct hot gases to heat-absorbing surfaces or toa separate means for further combustion, in which solid fuel particlesand combustion air are injected into said furnace and said fuel burnsreleasing gaseous products of combustion, in which noncombustibleparticles are present, in which said gases are induced to rotate aroundthe vertical axis and said rotation entrains at least some of said fueland noncombustible particles to rotate with said gases, and tan averageresidence time of said particles in said cylindrical enclosure isgreater than an average residence time of said gases in said cylindricalenclosure, and in which an air-to-fuel ratio is controlled at least atone combustion air injection location within said conically shapedbottom to control a temperature within said conically shaped bottom.

A twelfth embodiment, which is the furnace of the eleventh embodiment inwhich at least one melting temperature has been determined for at leastone composition of said noncombustible particles and said meltingtemperature is used to determine a desired range of said controllabletemperature.

A thirteenth embodiment, which is the furnace of the twelfth embodimentin which said conically shaped bottom is internally lined withrefractory and said controllable temperature is the temperature of saidrefractory measured at least at one location within said conicallyshaped bottom.

A fourteenth embodiment, which is the furnace of the twelfth embodimentin which said controllable temperature is a gas temperature measured atleast at one location above a top of said conically shaped bottom butbelow a lowest level of fuel injection.

A fifteenth embodiment, which is the furnace of the twelfth embodimentin which said controllable temperature is a temperature of saidnoncombustible particles extracted from said conically shaped bottom.

A sixteenth embodiment, which is the furnace of the twelfth embodimentin which said controllable temperature is a temperature of said gasmeasured within said conically shaped bottom.

A seventeenth embodiment, which is the furnace of the eleventhembodiment in which said air-to-fuel ratio is controlled by regulating aquantity of said combustion air injected at least at one of saidcombustion air injection locations.

An eighteenth embodiment, which is the furnace of the seventeenthembodiment in which the injection velocity of said combustion air iscontrolled independently of the flow of said combustion air.

A nineteenth embodiment, which is the furnace of the eleventh embodimentin which said air-to-fuel ratio is controlled by diluting saidcombustion air with oxygen-depleted gas taken from a furnace flue gas.

A twentieth embodiment, which is the furnace of the nineteenthembodiment in which an injection velocity of said diluted combustion airand said furnace flue gas is controlled independently of a flow of saiddiluted combustion air and said furnace flue gas.

A twenty-first embodiment, which is the furnace of the first embodimentin which said separate means for further combustion is an existingboiler.

A twenty-second embodiment, which is the furnace of the first embodimentin which said furnace is integrated into a stand-alone boiler.

A twenty-third embodiment, which is the furnace of the first embodimentin which said heat absorbing surfaces are contained in a separate heatrecovery steam generator.

A twenty-fourth embodiment, which is a furnace burning biomass fueldefined by a cylindrical enclosure with a vertical axis, a top of saidenclosure disposed to conduct hot gases to heat-absorbing surfaces or toa separate means for further combustion, in which solid fuel particlesand combustion air are injected into said furnace and said fuel burnsreleasing gaseous products of combustion, in which noncombustibleparticles are present, in which said gases are induced to rotate aroundthe vertical axis and said rotation entrains at least some of said fueland noncombustible particles to rotate with said gases, and an averageresidence time of said particles in said cylindrical enclosure isgreater than an average residence time of said gases in said cylindricalenclosure, and in which a momentum of said injected combustion air andfuel is controlled, either together or separately, to control a rotationof said rotating gases.

A twenty-fifth embodiment, which is the furnace of the twenty-fourthembodiment in which furnace flue gas is also injected, either separatelyor as a dilutant to said combustion air or as transport media for saidfuel, and in which a momentum of said injected furnace flue gas iscontrolled, either separately or together with said combustion air andfuel, to control a rotation of said rotating gases.

A twenty-sixth embodiment, which is a furnace burning biomass fueldefined by a cylindrical enclosure with a vertical axis, a top of saidenclosure disposed to conduct hot gases to heat-absorbing surfaces or toa separate means for further combustion, in which solid fuel particlesand combustion air are injected into said furnace and said fuel burnsreleasing gaseous products of combustion, in which noncombustibleparticles are present, in which said gases are induced to rotate aboutthe axis of said cylindrical enclosure and said rotation entrains atleast some of said fuel and noncombustible particles to rotate with saidgases, and an average residence time of said particles in saidcylindrical enclosure is greater than an average residence time of saidgases in said cylindrical enclosure, and in which a rotational velocityof said rotating gases is measured at least at one location within saidcylindrical enclosure, and in which said rotational velocity measurementis used as an input to a control loop to regulate said rotationalvelocity.

A twenty-seventh embodiment, which is a furnace burning biomass fueldefined by a cylindrical enclosure with a vertical axis, a bottom ofsaid enclosure conically shaped and truncated at a floor, a top of saidenclosure disposed to conduct hot gases to heat-absorbing surfaces or toa separate means for further combustion, in which solid fuel particlesand combustion air are injected into said furnace and said fuel burnsreleasing gaseous products of combustion, in which a portion of saidinjected fuel at least partially fills said bottom, in whichnoncombustible particles are present, in which said gases are induced torotate around the vertical axis and said rotation entrains at least someof said fuel and noncombustible particles to rotate with said gases, andan average residence time of said particles in said cylindricalenclosure is greater than an average residence time of said gases insaid cylindrical enclosure, in which said fuel residing in said bottomis induced to rotate about the vertical axis, and in which an injectionof combustion air, furnace flue gas, or steam, is regulated to control arotational velocity of said rotating fuel.

A twenty-eighth embodiment, which is the furnace of the twenty-seventhembodiment in which the rotational velocity of said rotating fuel ismeasured at least at one location within said conically shaped bottom,and in which said velocity measurement is used as an input to a controlloop to regulate said rotational velocity.

A twenty-ninth embodiment, which is the methods and apparatusesdescribed above.

A thirtieth embodiment, which is a method of operating a furnace,comprising injecting solid fuel particles and combustion air into acylindrical furnace; burning said fuel and releasing gaseous products ofcombustion and noncombustible particles, inducing said gases to rotatearound a vertical axis of said cylindrical furnace, wherein saidrotation entrains at least some of said fuel and noncombustibleparticles to rotate with said gases, and wherein an average residencetime of said particles in said cylindrical enclosure is greater than anaverage residence time of said gases in said cylindrical furnace; andcontrolling an air-to-fuel ratio at least at one combustion airinjection elevation to control a temperature above the elevation of saidcombustion air injection.

A thirty-first embodiment, which is the method of the thirtiethembodiment further comprising determining at least one meltingtemperature for at least one composition of said noncombustibleparticles and using said melting temperature to determine a desiredrange of said controllable temperature.

A thirty-second embodiment, which is the method of the thirty-firstembodiment in which said cylindrical furnace is internally lined withrefractory and controlling the controllable temperature is a temperatureof said refractory measured at least at one location within saidcylindrical furnace.

A thirty-third embodiment, which is the method of the thirty-firstembodiment in which said controllable temperature is a furnace exit gastemperature measured at least at one location at an exit of saidcylindrical furnace.

A thirty-fourth embodiment, which is the method of the thirty-firstembodiment in which said controllable temperature is a temperature ofsaid noncombustible particles extracted from said cylindrical furnace.

A thirty-fifth embodiment, which is the method of the thirty-firstembodiment in which said controllable temperature is a temperature ofsaid gas measured within a height of said cylindrical furnace.

A thirty-sixth embodiment, which is the method of the thirtiethembodiment in which said air-to-fuel ratio is controlled by regulating aquantity of said combustion air injected at least at one of saidcombustion air injection elevations.

A thirty-seventh embodiment, which is the method of the thirtiethembodiment in which said air-to-fuel ratio is controlled by dilutingsaid combustion air with oxygen-depleted gas taken from a furnace fluegas.

A thirty-eighth embodiment, which is the method of the thirty-seventhembodiment in which an injection velocity of said combustion air iscontrolled independently of a flow of said combustion air.

A thirty-ninth embodiment, which is the method of the thirty-eighthembodiment in which the injection velocity of said diluted combustionair and said furnace flue gas is controlled independently of the flow ofsaid diluted combustion air and said furnace flue gas.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the scope of the invention as defined by the appendedclaims. Moreover, the scope of the present application is not intendedto be limited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A furnace burning biomass fuel defined by a cylindrical enclosurewith a vertical axis, a top of said enclosure disposed to conduct hotgases to heat-absorbing surfaces or to a separate means for furthercombustion, in which solid fuel particles and combustion air areinjected into said furnace and said fuel burns releasing gaseousproducts of combustion, in which noncombustible particles are present,in which said gases are induced to rotate around the vertical axis andsaid rotation entrains at least some of said fuel and noncombustibleparticles to rotate with said gases, and an average residence time ofsaid particles in said cylindrical enclosure is greater than an averageresidence time of said gases in said cylindrical enclosure, and in whichan air-to-fuel ratio is controlled at least at one combustion airinjection elevation to control a temperature above the elevation of saidcombustion air injection.
 2. The furnace of claim 1 in which at leastone melting temperature has been determined for at least one compositionof said noncombustible particles and said melting temperature is used todetermine a desired range of said controllable temperature.
 3. Thefurnace of claim 2 in which said cylindrical enclosure is internallylined with refractory and said controllable temperature is thetemperature of said refractory measured at least at one location withinsaid cylindrical enclosure.
 4. The furnace of claim 2 in which saidcontrollable temperature is a furnace exit gas temperature measured atleast at one location at an exit of said cylindrical enclosure.
 5. Thefurnace of claim 2 in which said controllable temperature is atemperature of said noncombustible particles extracted from saidcylindrical furnace.
 6. The furnace of claim 2 in which saidcontrollable temperature is a temperature of said gas measured within acylindrical height of said furnace.
 7. The furnace of claim 1 in whichsaid air-to-fuel ratio is controlled by regulating a quantity of saidcombustion air injected at least at one of said air injectionelevations.
 8. The furnace of claim 7 in which an injection velocity ofsaid combustion air is controlled independently of a flow of saidcombustion air.
 9. The furnace of claim 1 in which said air-to-fuelratio is controlled by diluting said combustion air with oxygen-depletedgas taken from a furnace flue gas.
 10. The furnace of claim 9 in whichan injection velocity of said diluted combustion air and said furnaceflue gas is controlled independently of a flow of said dilutedcombustion air and said furnace flue gas.
 11. A furnace burning biomassfuel defined by a cylindrical enclosure with a vertical axis, a bottomof said enclosure being conically shaped and truncated at a floor, a topof said enclosure disposed to conduct hot gases to heat-absorbingsurfaces or to a separate means for further combustion, in which solidfuel particles and combustion air are injected into said furnace andsaid fuel burns releasing gaseous products of combustion, in whichnoncombustible particles are present, in which said gases are induced torotate around the vertical axis and said rotation entrains at least someof said fuel and noncombustible particles to rotate with said gases, andtan average residence time of said particles in said cylindricalenclosure is greater than an average residence time of said gases insaid cylindrical enclosure, and in which an air-to-fuel ratio iscontrolled at least at one combustion air injection location within saidconically shaped bottom to control a temperature within said conicallyshaped bottom.
 12. The furnace of claim 11 in which at least one meltingtemperature has been determined for at least one composition of saidnoncombustible particles and said melting temperature is used todetermine a desired range of said controllable temperature.
 13. Thefurnace of claim 12 in which said conically shaped bottom is internallylined with refractory and said controllable temperature is thetemperature of said refractory measured at least at one location withinsaid conically shaped bottom.
 14. The furnace of claim 12 in which saidcontrollable temperature is a gas temperature measured at least at onelocation above a top of said conically shaped bottom but below a lowestlevel of fuel injection.
 15. The furnace of claim 12 in which saidcontrollable temperature is a temperature of said noncombustibleparticles extracted from said conically shaped bottom.
 16. The furnaceof claim 12 in which said controllable temperature is a temperature ofsaid gas measured within said conically shaped bottom.
 17. The furnaceof claim 11 in which said air-to-fuel ratio is controlled by regulatinga quantity of said combustion air injected at least at one of saidcombustion air injection locations.
 18. The furnace of claim 17 in whichthe injection velocity of said combustion air is controlledindependently of the flow of said combustion air.
 19. The furnace ofclaim 11 in which said air-to-fuel ratio is controlled by diluting saidcombustion air with oxygen-depleted gas taken from a furnace flue gas.20. The furnace of claim 19 in which an injection velocity of saiddiluted combustion air and said furnace flue gas is controlledindependently of a flow of said diluted combustion air and said furnaceflue gas.
 21. The furnace of claim 1 in which said separate means forfurther combustion is an existing boiler.
 22. The furnace of claim 1 inwhich said furnace is integrated into a stand-alone boiler.
 23. Thefurnace of claim 1 in which said heat absorbing surfaces are containedin a separate heat recovery steam generator.
 24. A furnace burningbiomass fuel defined by a cylindrical enclosure with a vertical axis, atop of said enclosure disposed to conduct hot gases to heat-absorbingsurfaces or to a separate means for further combustion, in which solidfuel particles and combustion air are injected into said furnace andsaid fuel burns releasing gaseous products of combustion, in whichnoncombustible particles are present, in which said gases are induced torotate around the vertical axis and said rotation entrains at least someof said fuel and noncombustible particles to rotate with said gases, andan average residence time of said particles in said cylindricalenclosure is greater than an average residence time of said gases insaid cylindrical enclosure, and in which a momentum of said injectedcombustion air and fuel is controlled, either together or separately, tocontrol a rotation of said rotating gases.
 25. The furnace of claim 24in which furnace flue gas is also injected, either separately or as adilutant to said combustion air or as transport media for said fuel, andin which a momentum of said injected furnace flue gas is controlled,either separately or together with said combustion air and fuel, tocontrol a rotation of said rotating gases.
 26. A furnace burning biomassfuel defined by a cylindrical enclosure with a vertical axis, a top ofsaid enclosure disposed to conduct hot gases to heat-absorbing surfacesor to a separate means for further combustion, in which solid fuelparticles and combustion air are injected into said furnace and saidfuel burns releasing gaseous products of combustion, in whichnoncombustible particles are present, in which said gases are induced torotate about the axis of said cylindrical enclosure and said rotationentrains at least some of said fuel and noncombustible particles torotate with said gases, and an average residence time of said particlesin said cylindrical enclosure is greater than an average residence timeof said gases in said cylindrical enclosure, and in which a rotationalvelocity of said rotating gases is measured at least at one locationwithin said cylindrical enclosure, and in which said rotational velocitymeasurement is used as an input to a control loop to regulate saidrotational velocity.
 27. A furnace burning biomass fuel defined by acylindrical enclosure with a vertical axis, a bottom of said enclosureconically shaped and truncated at a floor, a top of said enclosuredisposed to conduct hot gases to heat-absorbing surfaces or to aseparate means for further combustion, in which solid fuel particles andcombustion air are injected into said furnace and said fuel burnsreleasing gaseous products of combustion, in which a portion of saidinjected fuel at least partially fills said bottom, in whichnoncombustible particles are present, in which said gases are induced torotate around the vertical axis and said rotation entrains at least someof said fuel and noncombustible particles to rotate with said gases, andan average residence time of said particles in said cylindricalenclosure is greater than an average residence time of said gases insaid cylindrical enclosure, in which said fuel residing in said bottomis induced to rotate about the vertical axis, and in which an injectionof combustion air, furnace flue gas, or steam, is regulated to control arotational velocity of said rotating fuel.
 28. The furnace of claim 27in which the rotational velocity of said rotating fuel is measured atleast at one location within said conically shaped bottom, and in whichsaid velocity measurement is used as an input to a control loop toregulate said rotational velocity.
 29. (canceled)
 30. A method ofoperating a furnace, comprising: injecting solid fuel particles andcombustion air into a cylindrical furnace; burning said fuel andreleasing gaseous products of combustion and noncombustible particles,inducing said gases to rotate around a vertical axis of said cylindricalfurnace, wherein said rotation entrains at least some of said fuel andnoncombustible particles to rotate with said gases, and wherein anaverage residence time of said particles in said cylindrical enclosureis greater than an average residence time of said gases in saidcylindrical furnace; and controlling an air-to-fuel ratio at least atone combustion air injection elevation to control a temperature abovethe elevation of said combustion air injection.
 31. The method of claim30 further comprising determining at least one melting temperature forat least one composition of said noncombustible particles and using saidmelting temperature to determine a desired range of said controllabletemperature.
 32. The method of claim 31 in which said cylindricalfurnace is internally lined with refractory and controlling thecontrollable temperature is a temperature of said refractory measured atleast at one location within said cylindrical furnace.
 33. The method ofclaim 31 in which said controllable temperature is a furnace exit gastemperature measured at least at one location at an exit of saidcylindrical furnace.
 34. The method of claim 31 in which saidcontrollable temperature is a temperature of said noncombustibleparticles extracted from said cylindrical furnace.
 35. The method ofclaim 31 in which said controllable temperature is a temperature of saidgas measured within a height of said cylindrical furnace.
 36. The methodof claim 30 in which said air-to-fuel ratio is controlled by regulatinga quantity of said combustion air injected at least at one of saidcombustion air injection elevations.
 37. The method of claim 30 in whichsaid air-to-fuel ratio is controlled by diluting said combustion airwith oxygen-depleted gas taken from a furnace flue gas.
 38. The methodof claim 37 in which an injection velocity of said combustion air iscontrolled independently of a flow of said combustion air.
 39. Themethod of claim 38 in which the injection velocity of said dilutedcombustion air and said furnace flue gas is controlled independently ofthe flow of said diluted combustion air and said furnace flue gas.